The present invention relates to an electrophotographic type image forming apparatus such as a copier, printer, facsimile device, or the like, in which a charging means, an image writing means and developing means are arranged around an image bearing body, and a toner image formed on the image bearing body is transferred onto a transfer material and fixed, and specifically relates to an image forming apparatus by which images can be formed on both sides of the transfer material.
Conventionally, in the double-sided image formation, a method is adopted in which a one side image formed on the image bearing body is transferred onto a transfer material and fixed; the transfer material is temporarily accommodated in an intermediate feeding device; the transfer material is fed from the intermediate feeding device in timed relationship with an image formed again on the image bearing body; and the image is transferred onto the other side of the transfer material and fixed.
As described above, in this double-sided image forming apparatus, the transfer material is fed to the intermediate feeding device and the transfer material is conveyed in such a manner that it passes trough the fixing device twice, thereby, reliability of the transfer material conveyance is low, resulting in a cause of jamming or wrinkling of the transfer material.
In contrast to this, a method in which, after toner images are formed on both sides of the transfer material using a first image bearing means and a second image bearing means, fixing is carried out only once, is proposed in Japanese Patent Publication Nos. 37538/1974, 28740/1979, Japanese Patent Publication Open to Public Inspection Nos. 44457/1989, and 214576/1992.
The present inventors are investigating an image forming apparatus and image forming method in which, after toner images are formed on both sides of the transfer material using a first image bearing means and a second image bearing means, fixing is carried out only once. In this connection, when this method is applied to the color image formation, double-sided color images are formed by the following method: plural sets of toner image forming means composed of a charging means, image writing means, developing means, etc., are arranged around a photoreceptor drum (the first image bearing means); after superimposed color toner images formed on the photoreceptor drum have been collectively transferred once onto a belt-like toner image receiving body (the second image bearing means) by a first transfer means, superimposed color toner images are formed again on the photoreceptor drum; a transfer material fed in timed relationship with the toner image on the photoreceptor drum and the toner image on the toner image receiving body, is electrically charged by a transfer material charging means and is attracted to the toner image receiving body; the toner image on the photoreceptor drum is transferred onto one side of the transfer material, conveyed by the toner image receiving body, as a front side image by the first transfer means and the toner image on the toner image receiving body is transferred onto the other side of the transfer material as a reverse side image by a second transfer material; after that, the transfer material is separated from the toner image receiving body by the curvature of a roller member which is provided on a side of a fixing device (fixing means) and around which the toner image receiving body is trained, and by electrical discharging of a transfer material separating means, which is provided as needed; and the toner image on the transfer material is fixed by the fixing device and the double-sided color images are formed.
However, in the doubled-sided image forming apparatus using the belt-like toner image receiving body as described above, the transfer material just after the back side toner image has been transferred onto the transfer material by the second transfer means, is electrostatically attracted to the toner image receiving body strongly, therefore, the transfer material is very hardly separated when the transfer material is separated from the toner image receiving body. Accordingly, in order to secure easy separation of the transfer material, the radius of curvature of the roller member at the end portion on the fixing device-side, around which the toner image receiving body is trained, is reduced and the curvature separation is carried out, or a transfer material separation means (AC discharger for paper separation) is provided for the electrostatic separation, however, there is a problem in which a separation failure occurs when environmental conditions are changed, or when thin paper is used.
Corresponding to this, it is considered that a separating claw to serve the separation of the transfer material from the second image bearing means is provided for ensuring the separation. However, there occurs a problem in which the transfer material after separation is in contact with the separation claw even while the transfer material is being fixed by the fixing means, therefore, an unfixed toner image on the transfer material, specifically the toner image on the reverse side is disturbed (rubbed), thereby, desirable double-sided images can not be formed. Reversely, a problem occurs in which unfixed toner adheres onto the separating claw when the back side image passes and the separation claw is stained.
Further, in the above image forming apparatus or image forming method, because the transfer material having unfixed toner images on both sides thereof is conveyed to the fixing device, the toner image receiving body is positioned adjacent to the fixing device, and the transfer material separated from the toner image receiving body is directly sent to the fixing device. However, when the toner image receiving body is positioned adjacent to the fixing device, problems occur in which the toner image receiving body is deformed by heat from the fixing device, or the toner image transferred onto the toner image receiving body is fused to some degree and thereby, sometimes transferring becomes difficult, or toner fixedly adheres onto the toner image receiving body. For this reason, it is considered that the second image bearing means and the fixing means are located apart from each other, and a conveyance means with a spurred wheel is located between them. However, in this case, there is a case in which the unfixed toner image on the transfer material, specifically the reverse side toner image is disturbed (rubbed), thereby, the double-sided images can not be finely formed. Further, depending on kinds of transfer materials, the transfer material separated from the toner image receiving body is not uniformly conveyed and conveyed irregularly, therefore, when the transfer material having unfixed toner images on both sides (also on the back side), which is separated from the toner image receiving body, is conveyed to the fixing device, there is also a problem in which the toner image on the reverse side is disturbed (rubbed), thereby, the double-sided images can not be finely formed.